Simplest Mistake
by Psylocke75
Summary: This a what if story, playing with an idea I cam up with, What if Mark and Addison had gotten married and Derek was the dirty Mistress! Chapter 4 now up! MERDER & MADDISON. This is going to be a fun one, I plotted it out.
1. Chapter 1

Dr. Mark Sloan unlocked the front door of his Manhattan home. He dropped his keys at the front door like he always did and removed his leather coat and went to hang it up on the antique coat rack, but he stopped. Hanging there was a smaller mans leather jacket, one he knew too well. The house had been dark, as if no one was home, but his wife's coat was also hanging there. He dropped his coat on the floor and slowly he walked up the stairs of his New York apartment. He paused on the top level, outside his bedroom door. He took a deep breath and prepared himself for the worst. He reached for the door handle, slowly he turned it, and then flung the door open. There in the middle of his bed was his wife and best friend, naked, panting starring right at him. Mark was torn, part of him wanted to close the door and walk away, but the primal male side wanted to rip the two of them a part and beat the shit of Derek Shepherd. The two sides compromised, he stood there calmly. "I think you should get your clothes and leave Derek, Addison I'll be in my study waiting for you." He closed the door. He stood there shaking for a second then slowly made his way downstairs to his study, where he poured himself a very large scotch.

Derek Shepherd put his clothes on, Addison Montgomery-Sloan just sat there on the bed, not saying anything. "Addi," he finally said.

"No Derek, just go, don't say anything, just go."

"I think I love you Addi," he said taking her hands. She pulled them away, "Don't say that, I'm married, to your best friend, we should never have started this."

"But we did, there must be something there."

"Derek I'm married to your best friend, you should just go."

Derek didn't know what else to say, he opened the door of the bedroom and made his way downstairs. He paused for a moment at Mark's study door. He wanted to go in and explain, but what do you say to your best friend after he finds you in bed with his wife. Derek did what he was told he grabbed his jacket and left.

Addison was shaking uncontrollably when she entered Mark's study. She didn't know what her husband would do, he had never been violent with her, but she had seen him loose his temper on occasion and every time it scared her. He was sitting on his big brown leather couch, and large glass of scotch in his hands. He looked up and told her to sit. "Here," he handed her the scotch.

"You know I don't drink."

"You need it, drink it."

She did as she told and took a huge swallow of the awful tasting liquid. She coughed a bit, but continued to drink it. When she was done she handed the empty glass back to her husband. Mark stood up a refilled the glass for himself. When he was at the small bar, he turned and asked, "Was this the first time?"

"No," she said honestly.

"How long?"

"Three weeks."

Mark just nodded, three weeks, how had he not noticed his wife sleeping with another man for three weeks. He turned back and looked at his lovely wife. She was the most beautiful woman he had ever seen, and the smartest. He had fallen in love with her the first moment he saw her in Gross Anatomy, but so had Derek Shepherd. Derek had told Mark he was going to ask her out, but Mark had stopped Derek, saying that he wanted to. They had fought at first about who was going to ask her out, but in the end Derek gave in and let Mark have a shot. Mark had never thought that there was a chance that Derek had been carrying a torch for Addison for all these years. It almost didn't seem possible. Derek had even been engaged, and he was devastated when she had ended it last year, claiming he was to involved with his work. Derek was the workaholic, the one that you had to almost beg to come to parties and put work aside. He was a brilliant Nero Surgeon, but Mark was an even more brilliant Plastic Surgeon, and Addi, well she was the best in her field, NeoNatal. Out of the three of them, Derek was the one that drove himself too hard, like he was trying to prove something. Derek hadn't grown up with the perks that Mark and Addison had. Derek came from a hard working family from Queens. His mom had been a nurse and his father was a Mechanic. He was always trying to put that behind him. He was like a man possessed with trying to be the best.

"Do you love him?"

Addison turned her head quickly to stare at her husband, "No, god no, I love you...I don't what started it."

"Was it revenge?"

"I forgave you for that Mark."

Addison didn't know why she started sleeping with Derek, maybe because she was depressed, she just didn't know why it started. Maybe part of herself hadn't completely forgave Mark for having an affair on her two years before, but it really hadn't gone through her mind until he had just mentioned it. She had no clue her husband was having an affair. It wasn't even an affair, he was away at a conference and had a one night stand with one of the waitresses. When he had gotten home he had confessed all, he had cried, which was the only time Addison had seen her husband cry. It had been unnerving for Addison to see such a big strong man break down and cry and beg for forgiveness. They had moved on, neither one ever talking about it again, almost like it never happened. There marriage had almost seemed stronger after that. Then three months ago Addison had asked Mark about starting a family. She was 36 years old, she wanted a baby. She delivered them everyday, but it wasn't the same as having your own. Mark told her he didn't want children, he wanted to spend their lives with just the two of them. Addison had tried to change his mind, but nothing had worked. Addison had gone into a deep depression. Mark had seem not to notice, being engrossed with work, friends, and his other past times. It was Derek that had noticed her mood, not her husband. She had spilled her heart out to him, and before she knew it they were back at his apartment having sex. It hadn't happen all that much, half a dozen times including tonight.

Why Addi, why then?"

He started to pace the room. "I don't know Mark."

"He was my best friend!" His voice had gotten louder, not yelling, but very close. Addison could see the adrenaline start to pump and he was getting angrier.

"I know, I'm sorry."

"You're sorry. That makes it all better? I've lost someone I loved like a brother. Tell me now if I've lost my wife too." He drank his drink fast. "Have I Addison? Have I lost my wife?" He was now yelling.

"No," she said barely above a whisper.

"What? I can't hear you."

"No Mark, No I love you!"

He threw his glass across the room and it smashed against the floor. Addison jumped and the sudden act of violence. Her large green eyes looked at her husband. She wasn't scared for herself, but she was suddenly very afraid for Derek. "I'm going out!"

"Where are you going?"

"Does it matter?"

"Yes. Don't hurt him Mark, blame me, not him."

"Don't worry baby I know where to lay the blame. I'll sleep in the spare room tonight, and tomorrow go to the Pottery Barn or wherever and buy a new bed, I want that one out of this apartment by the time I get home."

Mark grabbed Addison by the shoulders and nearly lifted her up. He kissed her hard, almost brutal, but it was a kiss. He let her go and stormed out of the apartment.


	2. Chapter 2

_My idea was always that Derek doesn't come from a wealthy background, but Mark and Addison did, so with out being married to money Derek never worked in his own private clinic._

Derek didn't drive directly home. He stopped at the Hospital and retrieved a letter that was in his office. It was a letter from one of his old attendings asking him to come be the head of their Nero department. It was a job of lifetime, it was everything Derek had worked towards, but it was on the other side of the country, in Seattle. When he first got the letter he almost disregarded it all together, but now, now he had nothing to lose. He'd be away from his mom and sisters, but he worked so much that he most of the time just talked to them on the phone. He was also being offered a lot more money then he was making in New York. Another perk was that Seattle Grace was a teaching hospital, the thought of molding young interns had its appeals. He retrieved the letter and then went home to his lonely dark apartment. His ex-fiance had taken most of the furniture and Derek hadn't bothered replacing much. His living room was almost empty, he kept his TV in the bedroom where also he had a small desk set up for his laptop.

Derek wasn't completely surprised to find Mark sitting at his small kitchen table when he walked in the door. A large bottle of scotch, that was about 3/4 full was sitting in front of him, he held an almost empty glass in his hands. He looked mad, but that was understandable. Most men would be if they caught their best friend in bed with their wives.

"I should never have given you a key." Derek said as he dropped his stuff at the front door.

"Do you want it back?" Mark asked as he topped up his glass.

"When I move I'll have to get it back." Derek walked into the kitchen.

"Your not moving."

Derek took the letter out of his pocket and slammed it on the table in front of Mark. As Mark read the letter Derek grabbed himself a glass and filled it. "Seriously, your going to Seattle?" Mark asked.

"I can't stay here."

"Why, because you were caught fucking my wife. You running Derek?"

"I'm not running, but that's a lot of money to give up" Derek sat down at the table. "Tell me why I should stay?"

"Family, friends, hell your from New York, you're like genetically engineered not to like any place that's not Manhattan."

"No that's you and Addi, Manhattan isn't home, Queens is my home." Derek leaned over the table and looked Mark in his eyes. "Why should I stayed, you said it, I fucked your wife, a few times. Not what a friend does."

"Do you want me to punch you, is that it. Trust me man I thought about it all the way over here." Mark refilled his glass.

"I want you to do something. If I was married to someone like Addi and caught my best friend in bed with her, I would hope to hell I would do something."

"I am doing something. I'm forgiving Derek. I don't want to loose my wife or someone I love like a brother."

"What if I love Addi?"

"She's easy to love."

"Stop it." Derek jumped up from the table. "I'm leaving. There's a chance I could be Chief one day. Here in New York I would never get the chance."

"Fine leave, if you think that's what you need to do, do it. You don't think I haven't noticed how hard you work, that's all you do Derek is work, maybe you can find happiness in Seattle."

Mark stood up from the table. "I should go. I meant it Derek I love you like I do my brothers."

When mark returned to his apartment, Addi was waiting up. She looked like a nervous fright. Neither one said anything to the other as she followed him upstairs to the spare bedroom. "Go to bed Addi," he told her.

"Did you hurt him?" she asked timidly.

"No. He's hurt himself enough," was all he said before he closed the door.


	3. Chapter 3

**Thanks for the reviews, and of course I don't have rights to Grey's Anatomy. Please keep the great reviews coming.**

**Chapter 3**

**2 Months later**

Derek was surprised at how sunny and warm Seattle could be when it stopped raining. It had rained everyday since he first arrived in Seattle two weeks before. He wasn't displeased about the change in weather, it made house hunting much easier and pleasant.

He still wasn't sure if her liked Seattle yet. He really hadn't had a chance to see much of the city since he arrived. Dr. Webber had literally thrown him to the wolves the minute he walked through the doors of Seattle Grace. This was the first day he even had a chance to look at houses. He wasn't what type of place he wanted, but the Realtor had a hot tip on a place that hadn't even gone on the market yet. The owner was out of town on for an indefinite amount of time, and her daughter had just moved back from Boston and was helping her mother with the sale. The daughter had agreed to let Derek come and see the house before it officially went up for sale.

Derek liked the small Arts and Crafts style house from the moment he stood on the front porch. He rang the front doorbell and as he waited he looked around at the veranda for any visible structural damage. The door opened while he had his back to it.

"Hello," a husky yet very sexy feminine voice said from behind him. Derek turned around and was speechless for a moment. She was tall and waif thin with long dark honey blond hair and large smokey grey blue eyes. She had to be one of the most attractive woman he had seen since landing in Seattle. She seemed amused by his dumb-fondness.

"Hi, Derek Shepherd, the realty company sent me," he finally said.

She held the door open wider and motioned for him to come in.

"Sorry for the mess, the house has been closed up for a few months," she said as he followed her into the living room..

Derek nodded, she was right, there were packed boxes all over and it was dusty. "No it's nice, Dusty but nice."

"I guess you want the full tour."

"Yes please," he said with a large smile.

She took him through the whole house, pointing out a few things here and there. "So did you grow up in this house?" Derek asked during the tour.

"It was my grandmothers, we moved in with her when my parents divorced, and when she died she left it to my mother."

"Oh I'm sorry."

"About what?"

"Your parents, your grandma, you know, it's what you say."

She smiled, and continued the tour. When they reached the upstairs she opened the first two smaller rooms. "They are about the same size, I think the back room has a larger closet."

She hesitated at the master bedroom. "I've been living here, until I can find a place, so please ignore any personal items."

She opened the room. It was dominated by a large queen sized bed. A few comfortable queen sized bed. For some reason he had mental images of him and her entwined naked on that large bed. He didn't know if he was blushing because when e looked over at her she also had a strange look to her face, as if she had been thinking the same thing.

"So that's the house," she said as she backed out of her room. "What do you think?"

"I think you should have dinner with me tonight."

"Seriously?" she asked and walked away down the stairs.

"Seriously have dinner with me," he said when caught up to her. "We're both new in town, lonely, why not?"

"I'm not new, I grew up in Seattle just moved back."

"See its perfect, you can show me around."

"No. Absolutely not."

"Why not."

"Because I'm not the kind of girl you want to take out for dinner."

"Come on," he begged a little, "have dinner with me. A nice steak, a couple bottles of wine. It'll be fun. Please."

"You don't strike me as the type that has to beg."

"Well I'm begging now."

"Fine come get me at 7."

"7 it is. I guess I need to ask you one more thing?"

"What?"

"What's your name?"

"Meredith."

"Meredith," he repeated her name and decided he liked it.


	4. Chapter 4

Chapter 4

Meredith surveyed herself in the mirror. She had no clue why she had agreed to dinner with Derek Shepherd. He was far too good looking with an amazing set of intense blue eyes, eyes that could be her downfall. She was right when she told him that she wasn't the type of girl he wanted to take out for dinner. Now if he had asked her if she wanted a bottle of Tequila and a one night stand, then she was his girl. Meredith knew she wasn't the most stable woman in the world. She didn't like ties of any type. Her entire sexual livelihood had been a series of one night stands. Only one had turned bad, and that was only a few short years ago when she was in Boston, and the result had been an abortion six weeks later. It wasn't something that she was proud of, but it was part of her past, a part of who she was, and just another reason why she shouldn't be having dinner with Derek Shepherd.

She double checked her watch, 10 to 7, he would be there any minute. She double checked her outfit, a lavender halter top and contrasting tulip skirt and a pair of strappy sandals. She knew she looked good, so she was ready to go.

Derek was there right at seven. Meredith greeted him with a large smile, but it faltered when he handed her a bouquet of pink tea roses. "Thank You," she whispered, she had never received flowers before, it was unnerving.

"You don't like them?" he asked.

"No, I do they're lovely. I have to see if I have anything to put them in." She left him at the front door as she went to see if she did indeed have anything in the kitchen. She was busy rummaging around in a kitchen cupboard when Derek came into the kitchen. She had found a good size tall measuring cup to put the roses in. When she turned around she jumped at seeing Derek a few feet behind her.

"I'm sorry I wanted to see if you were OK." he asked.

She smiled, slightly. "I'm fine, I just never have gotten flowers before."

"Well I'm glad I could be your first."

She blushed, "Well I think that's the only one of those you'll be," she laughed.

Derek had to chuckle, she was cute. He didn't know what possessed him to buy her flowers, he had never bought flowers for a date before, but Meredith was no way similar to any of the women he had dated back in New York. Most would have scoffed at such a trifle gesture. He had remembered his mother once saying that his father had brought a bouquet of tea roses on their first date. That image attached it self to his thoughts as he was getting dressed back at his hotel. He stopped at a flower shop on his way and bought her a bouquet.

He knew he had done the right thing when he saw that they had unnerved Meredith just that little bit. She was obviously a flustered with the way she was moving around the kitchen. She filled up the measuring cup and placed the roses in it, she turned to show him the masterpiece. "Very nice," he said.

"I have no clue why there was a measuring cup here, I don't think my mother ever cooked, but it could have been Grandma's," she said to him as she placed the flowers in the middle of the small table. As she had her back to Derek, he stepped closer to her, she turned around and gasped a little bit.

"Maybe we should do the goodnight kiss now, you know to get it over with so we can relax for the rest of the night."

Before Meredith could respond his lips were descending upon hers. It was a wonderful first kiss, and Meredith knew her first kisses, and this was first class.

When Derek pulled away he managed to form the words, "Now that's out of the way, let's go eat." He turned and walked out of the kitchen leaving a bewildered Meredith.

GAGAGAGAGAGAGAGAGAGAGAGAGAGAGAGAGA

Derek took her to small quiet restaurant that the Girl at the desk at the Hotel had suggested, perfect for a first date she had told him. They had a small table near the back, where they could hear each other talk, and not have to yell. "So you grew up in Seattle, your parents are divorced, your mother travels, your grandma's dead, you lived in Boston, so Meredith what else can you tell me."

"You know Derek, why don't we skip all the stuff, who and what we are outside of this room. I hate that first date bullshit, it doesn't matter."

"Then what do we talk about."

"I don't know," she laughed. "OK it was stupid suggestion."

"No it wasn't, it's just you take away all that and what do you have left, I like the idea of a little mystery, keeps it fun."

"Why did you want to ask me out?"

"Why did you say yes?"

The waitress set there drinks in front of them just as he asked. Meredith took a sip of her drink, and then said, "I asked first."

"I don't know, I mean your beautiful and you smell incredible, and your eyes and your hair are stunning, but your not like the women I dated back in New York. Now why did you say yes."

"That wasn't an answer."

"Well it's the truth. Why did you say yes."

"You begged."

"Liar. So why did you say no at first and tell me your not the type of girl I should have dinner with."

"To hear you beg," she laughed. "You want to know."

"Yes."

"I don't date, not very often, I am the girl that goes to the bar and gets drunk on Tequila and sleeps with inappropriate men, and then throws them out in the morning, or leaves before dawn. Only leaving memories, or sometimes an empty bottle of Tequila."

"Wow, so your messed."

"Yes. Yes I am."

"I'm not better. I was engaged once, and she moved out and I didn't see it coming until I came home one night, my place was empty, and she was gone."

"So we're a good couple."

"I think so. Should we order?"

"Yes, that would be good."

GAGAGAGAGAGAGAGAGAGAGAGAGAGAGAGAGA

After dinner Derek drove them to a spot where you could watch Ferry boats. He had found it a few days ago and wanted to go back, he had loved Ferry boats since he was a small child. "You cheated," She said when he told why he wanted to see the boats. That to her.

"What?" he asked. As he pulled out a car blanket from the back of his new Lexus.

"You cheated, I wasn't suppose to know what you like and don't like," she told him as he closed up the car.

"Well then in return you have to tell me something you've liked since you were a child." He walked to where there was a flat spot and laid the blanket out.

"I don't know. I can't think of anything."

"Seriously?" he asked as he sat down on the blanket and looked up at her.

"Well I feel like I'm on the spot here."

"Well you think about it while we sit here and watch the boats." He held his hand up to her, so she would sit down.

"You really just want to sit here and watch the Ferry boats." She sat down next to him.

"Oh yes, I have a pretty intense obsession with them."

"You're strange," she laughed.

"Why?"

"I don't know you just are."

"Is that bad?"

"I don't know yet."

Derek decided to try and kiss her again. He leaned close to her and hovered his lips above hers.

"What are you doing, we already had our goodnight kiss."

"This isn't goodnight," he said before setting in for a scorching kiss.

They had kissed for most of the night. By the time they gathered up the blanket and put it back in the car they were both extremely turned on, but neither one mentioned it. When he drove up to her house, he walked her up to her door, he wasn't sure if the evening would end there or continue on inside. As she unlocked the door and turned to face Derek. She reached up and kissed him. "I thought we already kissed goodnight," he said.

"Who says this is goodnight." She opened the door for him and motioned for him to come in.

He stood there for a minute debating on leaving, but she looked so perfect standing there, looking at him. He decided against his better judgment and went in the house.

Derek woke up early the next morning, he carefully untangled his naked body from Meredith's and dressed quietly dressed himself. He wanted to say goodbye, but decided not to, he knew he would see her again, but he had to get to the hospital, he wanted to stay, but knew he couldn't.

He leaned over and kissed her sleeping forehead and then left her room. On the way downstairs he noticed some mail left on the hall table, he knew he shouldn't look, but he wanted to at least know her full name, the mail was addressed to E. Grey, Meredith's mom he suspected. Meredith Grey, he tested the name out, and found it suited her, he liked it.


End file.
